Network congestion occurs when a link or node in the network is required to carry more data traffic than it is capable of transmitting or forwarding, with the result that its quality of service deteriorates. Typical effects of congestion include queuing delay, packet loss, and blocking of new connections. Modern packet networks use congestion control (including congestion avoidance) techniques in efforts to mitigate congestion before catastrophic results set in.
A number of congestion avoidance techniques are known in the art. In random early detection (RED, also known as random early discard or random early drop), for example, network nodes, such as switches, monitor their average queue size and drop packets based on statistical probabilities: If a given queue (or set of queues) is almost empty, all incoming packets are accepted. As the queue grows, the probability of dropping an incoming packet grows accordingly, reaching 100% when the buffer fill level passes the applicable threshold. Weighted RED (WRED) works in a similar fashion, except that different traffic classes are assigned different congestion avoidance thresholds, so that for a given queue length, low-priority packets have a greater probability of being dropped than high-priority packets. Congestion control techniques of this sort, which operate on a fraction of packets that is determined by statistical probabilities, are referred to herein as statistical congestion control techniques.
Another congestion avoidance technique is Explicit Congestion Notification (ECN), which is an extension to the Internet Protocol (IP) and the Transmission Control Protocol (TCP). ECN was initially defined by Ramakrishnan, et al., in “The Addition of Explicit Congestion Notification (ECN) to IP,” which was published as Request for Comments (RFC) 3168 of the Internet Engineering Task Force (2001) and is incorporated herein by reference. ECN provides end-to-end notification of network congestion by signaling impending congestion in the IP header of transmitted packets. The receiver of an ECN-marked packet of this sort echoes the congestion indication to the sender, which reduces its transmission rate as though it had detected a dropped packet. ECN functionality has recently been extended to other transport and tunneling protocols.